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Community and Q&A

Tape for foil-faced polyiso?

smokey059 | Posted in General Questions on

I need to seal the seams of foil faced polyiso. Can I use the aluminum foil tape they sell for hvac ducting. If not what type will work? Is the aluminum foil face tape for hvac ducting a vapor retardant or vapor barrier?

Or is it better to just leave a gap on the seams and then spray foam the seams?
If so how much gap is recommended? The foil face polyiso is vapor barrier so looking to get the seams the same.  Which way will give the best results? I’ve already asked about the vapour barrier  here in my wall assembly just wanting to get opinion on how best to seal the seams of polyiso  as that’s what I’m working on now. Its an interior application.

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  1. Expert Member
    Peter Engle | | #1

    In general, if the edges of your foam are in good shape and fit tightly, you don't need to spray foam them. If you are using reclaimed foam and it's a bit beat up, spray foam can help. Regardless, you also want to tape the seams with foil tape. The foil is a vapor barrier. Use a UL listed tape (it's printed all over the tape), and make sure to clean the surface of the foam first. Some of the UL listed foil-looking tape is actually plastic tape with a metallic finish. It might work, but I still like the actual foil.

  2. smokey059 | | #2

    Thank u for the reply. The tape I bought says ul listed aluminum foil tape. I tried a piece on the foil face and it sticks but it doesn't have the holding power of the 3m flashing tape I used on the plywood air barrier. I guess that might be normal for foil tape that it doesn't hold as powerfully as flashing tape but having never taped polyiso foil face that's an assumption on my part. It looks like foil tape and not plastic so I guess it must be the right stuff.

    1. Expert Member
      BILL WICHERS | | #3

      The “real” foil tape will feel a little like aluminum foil and will have a tendency to fold at sharp creases. The silver/plastic tapes tend to be more like “regular” tape and will resist making sharp creases.

      Try a quick wipe of the foil faced polyiso with some isopropyl alchohol, see if the tape sticks better after that. With a clean, dry foil surface, most adhesive tapes should hold pretty well — the “real” foil tapes should stick like crazy.


  3. smokey059 | | #4

    Thanks for the input. Ill try the isopropyl wipe to clean the surface before applying. It definitely feels like and creases like aluminum foil so I'm certain its the right stuff.

    Just hung one sheet to see how it would go and was also wondering should a person keep the cap nails in the field to avoid the tape going over them. And how many cap nails is needed (spacing) if 1x4 strapping is going over the polyiso for the sheetrock to attach too

  4. Expert Member
    Dana Dorsett | | #5

    Any temperature rated HVAC sealing tape is the right stuff. Nashua 324A is sold at most box stores, and fine for this application.

  5. Expert Member
    Peter Engle | | #6

    You only need enough nails to hold the polyiso until the furring goes on. Every 12"-16" should be fine. You can just put them in the field if you wish, but you will see that the polyiso sags a bit at the seams. That's not really a problem so long as you fasten the furring tightly.

    1. pakrat1 | | #7

      just wondering when the foam shrinks, foil tape won't stretch so my guess is that it will pull away leaving a gap for water entry especially if it separates at the top

      even though zip flashing tape is not reflective, I think it can stretch a small amount


      1. Expert Member
        Michael Maines | | #8

        Michael, I agree, and usually specify acrylic tape with polyiso because they are usually polyethylene or nylon based and can stretch. But where aesthetics matter, such as basement walls finished with white-faced polyiso, I use a white 3M foil tape. I have not seen or heard of any failures due to foam shrinkage but the seams are usually hidden so who knows.

      2. Expert Member
        BILL WICHERS | | #9

        I've never really gone back to check on this but I'd love some real-world info. When I apply the tape, the way I push it onto the surface ends up leaving a small depression in the gap, which means there is a tiny bit of extra tape "length" in there if it stretches, since the curve can straighten out a bit. I have no idea if this is enough to take care of any potential future shrinkage or not.

        I just recently replaced a bunch of foil faced polyiso on my house here (lots of damage so I couldn't reuse it), but unfortunately none of the seams were taped so I didn't get to see if the tape had failed. I can say that all the butted seams were still pretty tight together, and this was on a 40+ year old installation.


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